Electric power steering apparatus

ABSTRACT

An electric power steering apparatus ( 10 ) includes a steering wheel ( 21 ), a steering shaft ( 22 ), a rack-and-pinion mechanism ( 25 ), and a rack shaft ( 26 ) connected to the steering wheel through the steering shaft and the rack-and-pinion mechanism. The rack shaft is connected to vehicle wheels ( 29, 29 ). The electric power steering apparatus further includes a first electric motor ( 41 ) for producing a motive power to be applied to the steering shaft, and a second electric motor ( 51 ) for producing a motive power to be applied to the rack shaft.

TECHNICAL FIELD

The present invention relates to an improvement in an electric powersteering apparatus.

BACKGROUND ART

In recent years, many vehicles employ electric power steeringapparatuses for reducing steering forces with which drivers turnsteering wheels, so that the drivers steer the vehicles in comfort.These apparatuses are often used in large-sized vehicles as well. Forthe large-sized vehicles including the electric power steeringapparatuses, electric motors of the apparatuses unavoidably become largein size in order to produce greater assisting torques. However, theelectric motors should be sized to be disposed within small spaces ofthe vehicles. The vehicles must include circuits of large capacity fordriving the electric motors. Providing these circuits would lead toincreased cost for the vehicles.

To address the above problems, there have been developed electric powersteering apparatuses including two small-sized electric motors insteadof one large-sized electric motor. These electric power steeringapparatuses are known from, for example, Japanese Patent Laid-OpenPublication No. HEI 5-155343 entitled “ELECTRIC POWER STEERINGAPPARATUS”.

FIG. 26 hereof diagrammatically shows the electric power steeringapparatus disclosed in the publication.

The electric power steering apparatus designated generally at 700includes a steering wheel 701, a steering gear 702 connected to thesteering wheel 701, a rack shaft 703 connected to right and left vehiclewheels 711, 711, a steering torque sensor 704 attached to the gear 702,a control device 705 operable in response to operation of the sensor704, assisting motors 706, 707 to be controlled by the control device705, and steering gears 708, 709 to be driven by the motors 706, 707,respectively.

When the steering wheel 701 is turned by undergoing a steering torque,the steering gear 702 transmits the steering torque to the rack shaft703. Concurrently, the control device 705 drives the motors 706, 707 incorrespondence to the steering torque detected by the torque sensor 704.The motors 706, 707 then produce assisting torques corresponding to thesteering torque. The steering gears 708, 709 transmit the assistingtorques from the motors 706, 707 to the rack shaft 703. The rack shaft703 then turns the wheels 711, 711. Designated by reference numeral 712is a vehicle speed sensor.

The electric power steering apparatus 700 should become as small in sizeas possible for disposition within a small space of a vehicle. Reducingthe size of the electric power steering apparatus 700 requires takinginto account where to position the two electric motors. Accordingly,there is left a room for improvement in the electric power steeringapparatus.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an electric powersteering apparatus including two electric motors each disposed in such aposition as to reduce the size of the electric power steering apparatus.

According to a first aspect of the present invention, there is providedan electric power steering apparatus comprising: a steering wheel; arack shaft to be connected to vehicle wheels, the rack shaft beingconnected to the steering wheel through a steering shaft and arack-and-pinion mechanism; a first electric motor for producing a motivepower to be applied to the steering shaft or between the steering shaftand the rack-and-pinion mechanism; and a second electric motor forproducing a motive power to be applied to the rack shaft.

The motive power produced by the first electric motor is applied eitherto the steering shaft or between the steering shaft and therack-and-pinion shaft while the motive power produced by the secondelectric motor is applied to the rack shaft. The first and secondelectric motors are disposed at different positions far from each other.These electric motors can be more freely positioned within a vehiclebody. In other words, the electric motors can be positioned in a mannersuited to a space within a vehicle body. Accordingly, the vehicle bodycan easily provide a space where the electric motors are to bepositioned. The thus arranged electric power steering apparatus is smallin size.

According to a second aspect of the present invention, there is providedan electric power steering apparatus comprising: a steering wheel; arack shaft to be connected to vehicle wheels, the rack shaft beingconnected to the steering wheel through a steering shaft and arack-and-pinion mechanism; and two electric motors positioned adjacentand connected to the rack shaft through transmission mechanisms and aball screw.

The two electric motors are provided around the rack shaft fortransmitting to the vehicle wheels the steering torque applied to thesteering wheel. More specifically, the electric motors are connected viathe transmission mechanisms and the small-sized ball screw to the rackshaft. The two electric motors are positioned adjacent the rack shaft inclose proximity to each other. Since the electric motors are thuspositioned compactly, the electric power steering apparatus is small insize.

According to a third aspect of the present invention, there is providedan electric power steering apparatus comprising: a steering wheel; arack shaft to be connected to vehicle wheels, the rack shaft beingconnected to the steering wheel through a steering shaft, a pinion shaftand a rack-and-pinion mechanism having a pinion formed on the pinionshaft; and two electric motors for producing motive powers to be appliedto the pinion shaft, the two electric motors being connected to thepinion shaft with the pinion positioned therebetween.

The two electric motors are separately mounted or connected to thepinion shaft for transmitting to the rack, shaft the steering torqueapplied to the steering wheel. The pinion is interposed between the twoelectric motors. The motive powers produced by the two electric motorsare applied to the pinion shaft. The two electric motors are disposedadjacent the pinion shaft. Because the electric motors are compactlydisposed, the electric power steering apparatus is small in size.

According to a fourth aspect of the present invention, there is providedan electric power steering apparatus comprising: a steering wheel; arack shaft to be connected to vehicle wheels, the rack shaft beingconnected to the steering wheel through a steering shaft; a pinion shaftconnected to the rack shaft through a rack-and-pinion mechanism; a firstelectric motor for producing a motive power to be applied to the pinionshaft; and a second electric motor for producing a motive power to beapplied to the rack shaft through a ball screw.

The steering wheel is connected to the rack shaft through the steeringwheel. The pinion shaft is also connected to the rack shaft through therack-and-pinion mechanism. The first electric motor is provided forproducing the motive power to be applied to the pinion shaft while thesecond electric motor is provided for producing the motive power to beapplied to the rack shaft through the ball screw of small size. The twoelectric motors are positioned adjacent the rack shaft. Because theelectric motors are compactly disposed, the electric power steeringapparatus is small in size.

BRIEF DESCRIPTION OF DRAWINGS

Certain preferred embodiments of the present invention will hereinafterbe described in detail, by way of example only, with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic view of an electric power steering apparatusaccording to a first embodiment of the present invention;

FIG. 2 is a front elevation view of a housing of the apparatus of FIG.1;

FIG. 3 is a cross-sectional view taken along line 3-3 of FIG. 2;

FIG. 4 is a cross-sectional view taken along line 4-4 of FIG. 3;

FIG. 5 is a cross-sectional view taken along line 5-5 of FIG. 2;

FIG. 6 is a cross-sectional view taken along line 6-6 of FIG. 5;

FIG. 7 is a schematic view of an electric power steering apparatusaccording to a second embodiment of the present invention;

FIG. 8 a front elevation view of a housing of the apparatus of FIG. 7;

FIG. 9 is a view showing in cross-section the housing, a third electricmotor and a ball screw of the apparatus of FIG. 7;

FIG. 10 is a schematic view of an electric power steering apparatusaccording to a third embodiment of the present invention;

FIG. 11 is a perspective view of the apparatus of FIG. 10;

FIG. 12 is a cross-sectional view taken along line 12-12 of FIG. 11;

FIG. 13 is a cross-sectional view taken along line 13-13 of FIG. 12;

FIG. 14 is a schematic view of an electric power steering apparatusaccording to a fourth embodiment of the present invention;

FIG. 15 is a front elevation view showing a housing of the apparatus ofFIG. 14;

FIG. 16 is a cross-sectional view taken along line 16-16 of FIG. 15;

FIG. 17 is a view of a portion of the apparatus of FIG. 14 including thehousing, a ball screw, a rack shaft, first and second electric motorsand first and second belt transmission mechanisms, with the housing andthe ball screw fragmentarily shown in cross-section;

FIG. 18 is a schematic view of an electric power steering apparatusaccording to a fifth embodiment of the present invention;

FIG. 19 is a view of a portion of the apparatus of FIG. 18 including ahousing, the rack shaft, the first and second electric motors, the firstand second belt transmission mechanisms and a ball screw, with thehousing and the ball screw fragmentarily shown in cross-section;

FIG. 20 is a front elevation view of the housing and the first andsecond electric motors of the apparatus of FIG. 18;

FIG. 21 is a schematic view of an electric power steering apparatusaccording to a sixth embodiment of the present invention;

FIG. 22 is a front elevation view of a housing of the apparatus of FIG.21;

FIG. 23 is a cross-sectional view taken along line 23-23 of FIG. 22;

FIG. 24 is a schematic view of an electric power steering apparatusaccording to a seventh embodiment of the present invention;

FIG. 25 is a front elevation view of a housing of the apparatus of FIG.24; and

FIG. 26 is a conventional electric power steering apparatus;

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIG. 1 through FIG. 6, there is shown an electric powersteering apparatus 10 according to a first embodiment of the presentinvention. The apparatus 10 includes a steering system 20 and an assisttorque mechanism 30 for providing an assist torque to the steeringsystem 20.

As shown in FIG. 1, the steering system 20 includes a steering wheel 21,a steering shaft 22 attached at one end to the steering wheel 21, auniversal joint 23 having its one end connected to the other end of thesteering shaft 22, a pinion shaft 24 having its one end connected to theother end of the universal joint 23, and a rack shaft 26 a connected tothe other end of the pinion shaft 24 through a rack-and-pinion mechanism25. The rack shaft 26 a has a right end connected to one end of a righttie rod 27. The rack shaft 26 a has a left end connected to one end of aleft tie rod 27. The right tie rod 27 is connected via a knuckle 28 to aright vehicle wheel 29. The left tie rod 27 is connected via a knuckle28 to a left vehicle wheel 29. The right and left ends of the rack shaft26 a outputs a steering torque applied by a driver to the steering wheel21. The thus arranged apparatus 10 is often called “end-take-off-type”steering apparatus.

The rack-and-pinion mechanism 25 includes a pinion 25 a formed on thepinion shaft 24, and a rack 25 b formed on the rack shaft 26 a. Thepinion 25 a meshes with the rack 25 b.

When a driver applies the steering torque to the steering wheel 21 toturn the steering wheel, the steering torque is transmitted via therack-and-pinion mechanism 25, the rack shaft 26 a and the tie rods 27,27 to the right and left wheels 29, 29.

The assist torque mechanism 30 includes a steering torque sensor 31, acontrol section 32, and first and second assist torque producing devices40, 50. The sensor 31 is a magnetostrictive torque sensor.

When the sensor 31 detects the steering torque to produce a detectionsignal indicative of the detection of the steering torque, the controlsection 32 produces a motor controlling signal on the basis of thedetection signal. First and second electric motors 41, 51 produce assisttorques or motive powers corresponding to the steering torque on thebasis of the motor controlling signal. The motive powers produced by thefirst and second electric motors 41, 51 are transmitted to the wheels29, 29, as will be explained later.

The first assist torque producing device 40 includes the first electricmotor 41 and a first worm gear mechanism 42 serving as a torquetransmitting member for transmitting the assist torque produced by thefirst electric motor 41 to the pinion shaft 24.

The first worm gear mechanism 42 includes a first worm shaft 43 having afirst worm 44 formed thereon, and a first worm wheel 45 mounted on thepinion shaft 24. The first worm 44 meshes with the first worm wheel 45.The first electric motor 41 has a motor shaft 41 a connected via acoupling 46 to the first worm shaft 43.

The second assist torque producing device 50 includes the secondelectric motor 51, a second worm gear mechanism 52, an assist pinionshaft 57 and an assist rack-and-pinion mechanism 58. The second electricmotor 51 is connected to the rack shaft 26 a through the second wormgear mechanism 52, the assist pinion shaft 57 and the assistrack-and-pinion mechanism 58. The assist rack-and-pinion mechanism 58includes a pinion 58 a formed on the assist pinion shaft 57, and a rack58 b formed on the rack shaft 26 a. The pinion 58 a meshes with the rack58 b.

The second worm gear mechanism 52 includes a second worm shaft 53 havinga second worm 54 formed thereon, and a second worm wheel 55 mounted onthe assist pinion shaft 57. The second worm 54 meshes with the secondworm wheel 55. The second electric motor 51 has a motor shaft 51 aconnected via a coupling 56 to the second worm shaft 53.

The assist torque produced by the first electric motor 41 is transmittedthrough the coupling 46, the first worm gear mechanism 42, the pinionshaft 24 and the rack-and-pinion mechanism 25 to the rack shaft 26 a.The assist torque produced by the second electric motor 51 istransmitted through the coupling 56, the second worm gear mechanism 52,the assist pinion shaft 57 and the assist rack-and-pinion mechanism 58to the rack shaft 26 a.

Accordingly, the steering torque produced by the driver is combined withthe assist torques produced by the first and second electric motors 41,51 to thereby provide a composite torque. The composite torque isapplied to the rack shaft 26 a to thereby steer or turn the wheels 29,29.

As is apparent from the foregoing description, there is provided theelectric power steering apparatus 10 comprising: the steering wheel 22;the rack shaft 26 a to be connected to the vehicle wheels 29, 29, therack shaft 26 a being connected to the steering wheel 22 through thesteering shaft 22 and the rack-and-pinion mechanism 25; the firstelectric motor 41 for producing the motive power to be applied betweenthe steering shaft 22 and the rack-and-pinion mechanism 25; and thesecond electric motor 51 for producing the motive power to be applied tothe rack shaft 26 a.

As shown in FIG. 2, the rack shaft 26 a is accommodated within a housing61 a extending laterally of a vehicle body (not shown). The rack shaft26 a is slidable axially of the housing 61 a. The housing 61 a has anattachment portion 62 to be attached to the vehicle body. Referencenumerals 63, 64 denote a ball joint and a dust boot, respectively.

As shown in FIG. 3, the housing 61 a houses therein the rack-and-pinionmechanism 25 and the first worm gear mechanism 42. The housing 61 a hastwo top portions opened upwardly. One of the top portions is closed by alid 65. The other is closed by a lid 91 (see FIG. 2 or FIG. 5). Thepinion shaft 24 is inserted through the lid 65 into the housing 61 a.The steering torque sensor 31 is positioned proximate the pinion shaft24 within the lid 65.

The pinion shaft 24 has its upper, middle and lower portionsrotationally supported by three bearings 66, 67, 68, respectively. Thesebearings are vertically spaced from each other. The pinion shaft 24 hasthe middle and lower portions housed in the housing 61 a. The upperportion of the pinion shaft 24 is disposed within the lid 65. Thehousing 61 a has a rack guide 70A housed therein.

The rack guide 70A is provided for forcing a back surface portion of therack shaft 26 a opposite the rack 25 b. More specifically, the rackguide 70A includes a guide portion 71, an adjustment bolt 73, and acompression spring 72 interposed between the guide portion 71 and thebolt 73. The bolt 73 is disposed to cause the spring 72 to force theguide portion 71 against the back surface portion of the rack shaft 26a.

The bolt 73 is screwed into the housing 61 a, forcing the compressionspring 72 in such a direction as to push the guide portion 71 with aforce suitable for exerting a pre-load on the rack 25 b. The rack 25 bis pushed against the pinion 25 a, undergoing the pre-load to therebyestablish the meshing engagement with the pinion 25 b. Referencenumerals 69, 74, 75 denote an oil seal, a contact member and a lock nut,respectively. The rack shaft 26 a is slidable along the contact member74 with the back surface portion held in contact with the contact member74.

As shown in FIG. 4, the first electric motor 41 is attached to thehousing 61 a and has the motor shaft 41 a extending substantiallyhorizontally into the housing 61 a.

The first worm shaft 43 extending horizontally has its opposite endsrotationally supported by bearings 81, 82. The first worm shaft 43 isenclosed in a hollow eccentric sleeve 83 disposed within the housing 61a. Reference numerals 84, 85 designate nuts. Rotating the sleeve 83adjusts a backlash formed between the first worm 44 and the first wormwheel 45.

As shown in FIG. 5, the housing 61 a houses therein the second worm gearmechanism 52 and the assist rack-and-pinion mechanism 58. The housing 61a has the top portion closed by the lid 91.

The assist pinion shaft 57 has its upper, middle and lower portionsrotationally supported by bearings 92, 93, 94, respectively. Thesebearings 92, 93, 94 are vertically spaced from each other. The upperportion of the assist pinion shaft 57 is positioned above the topportion of the housing 61 a and within the lid 91. The assist pinionshaft 57 has the middle and lower portions housed in the housing 61 a.The housing 61 a has a rack guide 70B housed therein.

The rack guide 70B is the same in construction as the rack guide 70A.That is, the rack guide 70B is provided for forcing a back surfaceportion of the rack shaft 26 a opposite the rack 58 b. Components of therack guide 70B are denoted by the same reference numerals as the rackguide 70A.

As shown in FIG. 6, the second electric motor 51 is attached to thehousing 61 a and has the motor shaft 51 a substantially horizontallyextending into the housing 61 a.

The second worm shaft 53 extending horizontally has its opposite endsrotationally supported by bearings 95, 96. The second worm shaft 53 isenclosed in a hollow eccentric sleeve 97 disposed within the housing 61a. Reference numerals 98, 99 designate nuts. Rotating the sleeve 97adjusts a backlash formed between the second worm 54 and the second wormwheel 55.

As discussed with reference to FIG. 1 through FIG. 6, the first electricmotor 41 produces a motive power to be applied to the pinion shaft 24disposed between the steering shaft 22 and the rack-and-pinion mechanism25 while the second electric motor 51 produces a motive power to beapplied to the rack shaft 26 a. The first and second electric motors 41,51 are positioned away from each other. The electric power steeringapparatus 10 has the advantage that the electric motors can be morefreely positioned in a manner suited to a space within the vehicle body.This makes it easier to determine a space where the electric motors arepositioned. In addition, the thus arranged electric power steeringapparatus 10 can be small in size.

Furthermore, the rack-and-pinion mechanism 25 for transmitting to therack shaft 26 a the steering torque applied to the steering wheel 21 andthe assist torque produced by the first electric motor 41 is notsubjected to the motive power supplied from the second electric motor51. This arrangement helps the rack-and-pinion mechanism 25 maintainsufficient strength.

Referring to FIG. 7 through FIG. 9, there is shown an electric powersteering apparatus 100 according to a second embodiment of the presentinvention. The electric power steering apparatus 100 is the same as theelectric power steering apparatus 10 except that the assist torquemechanism 30 includes a third assist torque producing device 150 insteadof the second assist torque producing device 50. In FIG. 7 to FIG. 9,the same components of the electric power steering apparatus 100 asthose of the electric power steering apparatus 10 are identicallynumbered and their detail description will be omitted.

More specifically, the assist torque mechanism 30 of the apparatus 100includes the steering torque sensor 31, the control section 32, and thefirst and third assist torque producing devices 40, 150. The thirdassist torque producing device 150 includes a third electric motor 151and a ball screw 160 connecting the motor 151 to a rack shaft 26 btherethrough.

As show in FIG. 7, the rack shaft 26 b has the rack 25 b formed on oneend thereof. In addition to the rack 25 b, the rack shaft 26 b has athreaded portion 161 formed thereon. The rack 25 b and the threadedportion 161 are provided separately from each other. The ball screw 160has a nut 163 mounted on the threaded portion 161 as will be describedlater. The third electric motor 151 is hollowed to allow the rack shaft26 b to extend therethrough. The third electric motor 151 is connectedto the nut 163. The nut 163 is positioned between the rack 25 b and thethird electric motor 151. The third electric motor 151 is often called,“coaxial motor”.

On the basis of the motor controlling signal output from the controlsection 32, the first and third electric motors 41, 151 produce assisttorques (motive powers) corresponding to the steering torque. The assisttorque produced by the third electric motor 151 is transmitted throughthe ball screw 160 to the rack shaft 26 b. The steering torque thedriver applies to the steering wheel 21 and the assist torques producedby the first and third electric motors 41, 151 are combined together toprovide a composite torque. The composite torque is transmitted to therack shaft 26 b to thereby steer or turn the wheels 29, 29.

In the electric power steering apparatus 100 according to the secondembodiment of the present invention, the steering wheel 21 is connectedthrough the steering shaft 22 and the rack-and-pinion mechanism 25 tothe rack shaft 26 b. The rack shaft 26 b is connected to the wheels 29,29. The first electric motor 41 produces the motive power to be appliedto the pinion shaft 24 disposed between the steering shaft 22 and therack-and-pinion mechanism 25 while the third electric motor 151 producesthe motive power to be applied to the rack shaft 26 b.

As shown in FIG. 8, the electric power steering apparatus 100 includes ahousing 61 b. The housing 61 b includes first and second housingportions 171, 172 of generally tubular configuration. The first housingportion 171 has one end bolted to one end of the second housing portion172, thereby providing the housing 61 b with an elongated gear boxconfiguration. The second housing 172 serves as a motor case enclosingthe third electric motor 151 therein.

It is to be understood that the first assist torque producing device 40is arranged in the manner as described in relation to FIG. 3 and FIG. 4.FIG. 8 provides a cross-sectional view of FIG. 3 taken along line 3A-3Athereof to show the first assist torque producing device 40 incross-section.

As shown in FIG. 9, the third electric motor 151 includes a cylindricalstator 152 fitted within the second housing portion 172, a rotor 153disposed inside the stator 152, and a tubular motor shaft 154 unitedwith the rotor 153. The motor shaft 154 is hollowed to allow the rackshaft 26 b to extend therethrough. The motor shaft 154 is rotatablerelative to the rack shaft 26 b. The motor shaft 154 has its innerdiameter greater than an outer diameter of the threaded portion 161.

The ball screw 160 is of conventional type and includes the threadedportion 161 serving as an external thread formed on the rack shaft 26 b,plural balls 162 disposed on a peripheral surface of the threadedportion 161, and the nut 163 mounted on the balls 162. The nut 163serves as an outer cylindrical member hollowed to allow the threadedportion 161 to extend therethrough. The nut 163 and the balls 162 of theball screw 160 are capable of transmitting to the threaded portion 161the assist torque produced by the third electric motor 151. The nut 163has a threaded portion or a groove formed along an inner surfacethereof. The groove is in the form of a helix. The balls 162 are guidedalong the groove. The nut 163 has a tube (not shown) formed therein. Thetube extends between opposite ends of the groove. The tube is providedfor guiding the respective balls, which have been guided to the one endof the groove, to the other end of the groove.

The first housing portion 171 includes a ball screw housing portion 171a fitted into the second housing portion 172. The ball screw housingportion 171 a supports the nut 163 by means of a first bearing 181 suchthat the nut 163 is rotatable but unmovable axially. The nut 163 has aconnection aperture 163 a formed separately from the groove. The motorshaft 154 has an output end 154 a fitted into the connection aperture163 a in such a manner as to transmit the motive power of the thirdelectric motor 151 to the nut 163.

The output end 154 a of the motor shaft 154 and the nut 163 arerotationally supported by the first bearing 181 within the first housingportion 171. The motor shaft 154 has another output end 154 b oppositethe output end 154 a. The output end 154 b is rotationally supported bya second first bearing 182 within the second housing portion 172.

As discussed with reference to FIG. 7 through FIG. 9, the first electricmotor 41 produces the motive power to be applied to the pinion shaft 24disposed between the steering shaft 22 and the rack-and-pinion mechanism25 while the third electric motor 151 produces the motive power to beapplied to the rack shaft 26 b. The first and third electric motors 41,151 are provided away from each other. The electric power steeringapparatus 100 has the advantage that the electric motors can be morefreely positioned in a manner suited to a space within the vehicle body.This makes it easier to determine a space where the electric motors areto be positioned. In addition, the electric power steering apparatus 100can be small in size.

The electric power steering apparatus 100 includes the third electricmotor 151 for producing the motive power to be applied to the rack shaft26 b through the ball screw 160 of small size. This makes it much easierto determine a space where the electric motors 41, 151 are positioned.Additionally, the electric power steering apparatus 100 can be small insize. Furthermore, there is no need for the aforementioned assistrack-and-pinion mechanism, assist pinion shaft and rack guide fortransmitting the motive power of the third electric motor 151 to therack shaft 26 b. This is advantageous in that the rack shaft 26 b can beshortened by a length along which the assist pinion shaft and the assistrack-and-pinion mechanism would be otherwise disposed. This leads toreduced cost for the rack shaft 26 b.

Moreover, the transmission member, that is, the rack-and-pinionmechanism 25 for transmitting to the rack shaft 26 b the steering torqueapplied to the steering wheel 21 and the assist torque produced by thefirst electric motor 41 is not subjected to the motive power suppliedfrom the third electric motor 151. This arrangement helps therack-and-pinion mechanism 25 maintain sufficient strength.

Referring to FIG. 10 through FIG. 13, there is shown an electric powersteering apparatus 200 according to a third embodiment of the presentinvention.

As shown in FIG. 10, the electric power steering apparatus 200 is thesame as the electric power steering apparatus 100 except that the assisttorque mechanism 30 includes a fourth assist torque producing device 240instead of the first assist torque producing device 40. The fourthassist torque producing device 240 includes the first electric motor 41disposed in such a position to provide a motive power to the steeringshaft 22. In FIG. 10 to FIG. 13, the same components of the electricpower steering apparatus 200 as those of the electric power steeringapparatus 100 are identically numbered and their detail descriptionswill be omitted.

The fourth assist torque producing device 240 includes the firstelectric motor 41 and the first worm gear mechanism 42 for transmittingto the steering shaft 22 the assist torque produced by the firstelectric motor 41. The first worm gear mechanism 42 includes the firstworm shaft 43 having the first worm 44 formed thereon, and the firstworm wheel 45 mounted on the steering shaft 22. The first worm 44 mesheswith the first worm wheel 45.

As shown in FIG. 11, the electric power steering apparatus 200 includesthe steering system 20 and the third and fourth assist torque producingdevices 150, 240.

As shown in FIG. 12, the electric power steering apparatus 200 includesthe steering shaft 22, the steering torque sensor 31 and the first wormgear mechanism 42. The first worm gear mechanism 42 is housed in ahousing 261. The housing 261 has a top portion opened upwardly. The topportion is closed by a lid 265. The steering shaft 22 extends throughthe lid 265 and the housing 261. The steering torque sensor 31 isdisposed proximate the steering shaft 22.

The steering shaft 22 has its upper and lower portions rotationallysupported by upper and lower bearings 266, 267, respectively. The upperportion of the steering shaft 22 is disposed within the lid 265 whilethe lower portion of the steering shaft 22 is disposed within thehousing 261. Reference numerals 268, 269 denote oil seals.

FIG. 12 provides a cross-sectional view of FIG. 4 taken along line 4A-4Athereof to show in cross-section the first worm gear mechanism 42 withthe pinion shaft 24 replaced with the steering shaft 22.

FIG. 13 is the view of FIG. 3 with the steering torque sensor 31 and thefirst worm gear mechanism 42 removed.

As discussed with reference to FIG. 10 through FIG. 13, the firstelectric motor 41 produces the motive power to be applied to thesteering shaft 22 while the third electric motor 151 produces the motivepower to be applied to the rack shaft 26 b. The first and third electricmotors 41, 151 are positioned away from each other. The electric powersteering apparatus 200 has the advantage that the electric motors can bemore freely positioned in a manner suited to a space within the vehiclebody. This makes it easier to determine a space where the electricmotors are positioned. In addition, the electric power steeringapparatus 200 can be small in size.

Further, the pinion shaft 24 has no electric motor mounted thereto. Inother words, around the pinion shaft 24, there can be formed a freespace which would be otherwise occupied by the electric motors. Such aspace can be advantageously utilized for other purposes.

As is apparent from FIG. 10 and FIG. 11, the electric power steeringapparatus 200 includes the third electric motor 151 for producing themotive power to be transmitted via the ball screw 160 of small size tothe rack shaft 26 b. This makes it much easier for the vehicle toprovide a space where the electric motors 41, 151 are positioned. Theelectric power steering apparatus 200 can be made small in size.Furthermore, there is no need for any assist rack-and-pinion mechanism,assist pinion shaft and rack guide for transmitting the motive power ofthe third electric motor 151 to the rack shaft 26 b. This isadvantageous in that the rack shaft 26 b can be shortened by a lengthalong which the assist pinion shaft and the assist rack-and-pinionmechanism would be otherwise disposed. This leads to reduced cost forthe rack shaft 26 b.

Moreover, the transmission member, that is, the rack-and-pinionmechanism 25 for transmitting to the rack shaft 26 b the steering torqueapplied to the steering wheel 21 and the assist torque produced by thefirst electric motor 41 is not subjected to the motive power suppliedfrom the third electric motor 151. This arrangement helps therack-and-pinion mechanism 25 maintain sufficient strength.

Referring to FIG. 14 through FIG. 17, there is shown an electric powersteering apparatus 300 according to a fourth embodiment of the presentinvention. The electric power steering apparatus 300 is identical to theelectric power steering apparatus 10 except the assist torque mechanism30 is replaced with an assist torque mechanism 330, as will be describedlater. In FIG. 14 to FIG. 17, the same components of the apparatus 300as those of the apparatus 10 are identically numbered and their detaildescriptions will be omitted.

As shown in FIG. 14, when the sensor 31 detects the steering torque toproduce the detection signal, the control section 32 produces the motorcontrolling signal on the basis of the detection signal. The first andsecond electric motors 41, 51 produce the assist torques or motivepowers corresponding to the steering torque on the basis of the motorcontrolling signal. The motive powers produced by the first and secondelectric motors 41, 51 are transmitted to a rack shaft 26 c for turningthe wheels 29, 29. The rack shaft 26 c has the rack 25 b formed thereon.

The assist torque mechanism 330 includes the steering torque sensor 31,the control section 32 and first and second assist torque producingdevices 340, 350.

The first assist torque producing device 340 includes the first electricmotor 41, a first belt transmission mechanism 341 and a ball screw 360.The first belt transmission mechanism 341 and the ball screw 360 arearranged to transmit the assist torque produced by the first electricmotor 41 to the rack shaft 26 c.

The second assist torque producing device 350 includes the secondelectric motor 51, a second belt transmission mechanism 351 and the ballscrew 360. The second belt transmission mechanism 351 and the ball screw360 are arranged to transmit the assist torque produced by the secondelectric motor 51 to the rack shaft 26 c.

The assist torques produced by the first and second electric motors 41,51 are transmitted via the ball screw 360 to the rack shaft 26 c. Theassist torques produced by the first and second electric motors 41, 51and the steering torque produced by the driver are combined together toprovide a composite torque. The composite torque is transmitted to therack shaft 26 c to thereby steer or turn the wheels 29, 29.

As is apparent from the foregoing description, there is provided theelectric power steering apparatus 300 comprising: the steering wheel 21;the rack shaft 26 c to be connected to the vehicle wheels 29, 29, therack shaft 26 c being connected to the steering wheel 21 through thesteering shaft 22 and the rack-and-pinion mechanism 25; and the twoelectric motors 41, 51 positioned adjacent and connected to the rackshaft 26 c through the transmission mechanisms 341, 351 and the ballscrew 360.

As shown in FIG. 15, the rack-and-pinion mechanism 25 and the ball screw360 are housed in a housing 61 c. The housing 61 c has the first andsecond electric motors 41, 51 disposed along an outer peripheral surfacethereof.

The housing 61 c includes first and second housing portions 371, 372.The first housing portion 371 has one end bolted to one end of thesecond housing portion 372 to thereby provide the housing 61 c with anelongated gear box configuration. The housing 61 c extends laterally ofthe vehicle body.

FIG. 16 is the view of FIG. 3 with the first worm gear mechanism 42removed.

As shown in FIG. 17, more specifically, the first housing portion 371has a flange 371 a formed at the one end thereof while the secondhousing portion 372 has a flange 372 a formed at the one end thereof.The flanges 371 a, 372 a are bolted together.

The first and second electric motors 41, 51 extend in parallel to therack shaft 26 c. More specifically, the first and second electric motors41, 51 are disposed along the outer peripheral surface of the housing 61c with the motor shaft 41 a and the motor shaft 51 a extending in thesame direction in parallel to the rack shaft 26 c. The first and secondelectric motors 41, 51 are attached to the second housing portion 372.More specifically, the first and second electric motors 41, 51 areattached to the flange 372 a. In FIG. 17, the first electric motor 41 ispositioned above the rack shaft 26 c while the second electric motor 51is positioned below the rack shaft 26 c. Also, the first and secondelectric motors 41, 51 are disposed in substantially vertical alignmentwith each other.

The first belt transmission mechanism 341 includes a first drivingpulley 342 mounted or connected to the motor shaft 41 a of the firstelectric motor 41, a first driven pulley 343 joined or mounted to a nut363 of the ball screw 360, and a first belt 344 running over the pulleys342, 343.

The second belt transmission mechanism 351 includes a second drivingpulley 352 mounted or connected to the motor shaft 51 a of the secondelectric motor 51, a second driven pulley 353 joined or mounted to thenut 363 of the ball screw 360, and a second belt 354 running over thepulleys 352, 353.

As shown in this figure, the mechanisms 341, 351 are provided alongsidethe flange 372 a. As can be seen from this figure, the mechanism 341disposed above the nut 363 and the rack 26 c. The mechanism 351 isdisposed below the nut 363 and the rack shaft 26 c. The pulleys 343, 353are positioned between the flange 372 a and a bearing 381. The pulleys343, 353 are juxtaposed longitudinally of the nut 363. The pulley 353 ispositioned closer to the bearing 381 than the pulley 343.

The ball screw 360 is of conventional type and includes a threadedportion 361 serving as an external thread formed on the rack shaft 26 c,plural balls 362 disposed on a peripheral surface of the threadedportion 361, and the nut 363 mounted on the balls 362. The nut 363serves as an outer cylindrical member hollowed to allow the threadedportion 361 to extend therethrough. The nut 363 and the balls 362 arecapable of transmitting to the threaded portion 361 the assist torquesproduced by the first and second electric motors 41, 51. The nut 363 hasa threaded portion or a groove formed along an inner surface thereof.The groove is in the form of a helix. The balls 362 are guided along thegroove. The nut 363 has a tube (not shown) formed therein. The tubeextends between opposite ends of the groove. The tube is provided forguiding the respective balls, which have been guided to the one end ofthe groove, to the other end of the groove.

The first housing portion 371 includes a ball screw housing portion 371b integral with the flange 371 a. The ball screw housing portion 371 bsupports the nut 363 by means of the bearing 381 such that the nut 363is rotatable but unmovable axially.

As discussed with reference to FIG. 14 to FIG. 17, the electric powersteering apparatus 300 includes the two electric motors 41, 51 disposedaround the rack shaft 26 c for transmitting to the wheels 29, 29 thesteering torque applied to the steering wheel 21 and the assist torquesproduced by the electric motors 41, 51. The electric motors 41, 51 areconnected through the mechanisms 341, 351 and the single ball screw 360of small size to the rack shaft 26 c. The two electric motors 41, 51 arepositioned adjacent the rack shaft 26 c in close proximity to eachother. In other words, the electric motors 41, 51 can be providedcompactly. The electric power steering apparatus 300 can be made smallin size and therefore occupy a smaller space within the vehicle body.

Further, there is no need to provide any electric motor between thesteering shaft 22 and the rack-and-pinion mechanism 25. In other words,between the steering shaft 22 and the rack-and-pinion mechanism 25,there can be formed a free space within which the electric motor wouldbe otherwise positioned. Such a space can be used for other purposes.

Because the two electric motors 41, 51 are connected via the single ballscrew 360 to the rack shaft 26 c, no rack-and-pinion mechanism forconnecting the electric motors 41, 51 to the rack shaft 26 c isrequired. Accordingly, there is no need for any rack guide other thanthe rack guide 70A. The electric power steering apparatus 300 thusarranged is simple in construction.

The electric motors 41, 51 are connected through the belt transmissionmechanisms 341, 351 and the ball screw 360 to the rack shaft 26 c. Thismakes it possible to provide a reduced distance between the rack shaft26 c and each of the motor shafts 41 a, 51 a. The electric motors 41, 51can thus also be positioned more freely.

The electric motors 41, 51 are well-known, less expensive ones ratherthan the aforementioned coaxial motors.

Referring to FIG. 18 through FIG. 20, there is shown an electric powersteering apparatus 400 according to a fifth embodiment of the presentinvention. The electric power steering apparatus 400 is the same as theelectric power steering apparatus 300 except that the electric motors41, 51 are positioned in different positions from those in the fourthembodiment. In FIG. 18 to FIG. 20, the same components of the apparatus400 as those of the electric power steering apparatus 300 areidentically numbered and their detail descriptions will be omitted.

As shown in FIG. 18, the steering wheel 21 is connected via the steeringshaft 22 and the rack-and-pinion mechanism 25 to the rack shaft 26 c.The rack shaft 26 c is connected to the wheels 29, 29. The two electricmotors 41, 51 are disposed around the rack shaft 26 c. Morespecifically, these electric motors 41, 51 are connected through thefirst and second belt transmission mechanisms 341, 351 and a single ballscrew 360′ to the rack shaft 26 c. The ball screw 360′ includes thethreaded portion 361, the plural balls 362, and a nut 363′ mounted onthe balls 362.

As shown in FIG. 19, the motor shaft 41 a has its one end positioned inopposed relation to one end of the motor shaft 51 a. The motor shafts 41a, 51 a are parallel to the rack shaft 26 c. The first and secondelectric motors 41, 51 are disposed along an outer peripheral surface ofa housing 61 c′. The electric motors 41, 51 extend in oppositedirections and are disposed in parallel to the rack shaft 26 c. Thehousing 61 c′ includes first and second housing portions 371′, 372′. Thefirst electric motor 41 is attached to the first housing portion 371′whilst the second electric motor 51 is attached to the second housingportion 372′.

The first housing portion 371′ has a flange 371 a′ formed at the one endthereof while the second housing portion 372′ has a flange 372 a′ formedat the one end thereof. The flanges 371 a′, 372 a′ have an intermediateflange 373 interposed therebetween. The intermediate flange 373 isbolted to the flanges 371 a′, 372 a′. The intermediate housing 373supports the nut 363′ by means of a bearing 381′ so that the nut 363′ isrotatable but unmovable axially. The first electric motor 41 is attachedto the flange 371 a′. The second electric motor 51 is attached to theflange 372 a′. As can be seen from this figure, the flanges 371 a′, 372a′ are positioned above the rack shaft 26 c. The first driven pulley 343is joined or mounted to the nut 363′ of the ball screw 360′. Likewise,the second driven pulley 353 is joined or mounted to the nut 363′. Asshown in FIG. 19, the mechanisms 341, 351 are provided above the nut363′ and the rack shaft 26 c. The mechanism 341 is provided alongsidethe flange 371 a′. The mechanism 351 is provided alongside the flange372 a′. The pulleys 343, 353 have the bearing 381′ positionedtherebetween.

As shown in FIG. 20, the first and second electric motors 41, 51 arehorizontally aligned with each other and disposed along the outerperipheral surface of the housing 61 c′.

As described with reference to FIG. 18 through FIG. 20, the two electricmotors 41, 51 are provided above the rack shaft 26 c for transmitting tothe wheels 29, 29 the steering torque applied to the steering wheel 21and the assist torques produced by the electric motors 41, 51. Morespecifically, the electric motors 41, 51 are connected via thetransmission mechanisms 341, 351 and the ball screw 360′ of small sizeto the rack shaft 26 c. The electric motors 41, 51 are positionedadjacent the rack shaft 26 c in close proximity to each other. In otherwords, the electric motors 41, 51 are positioned compactly. The electricpower steering apparatus 400 thus arranged are small in size.

Further, there is no need to provide any electric motor between thesteering shaft 22 and the rack-and-pinion mechanism 25. In other words,between the steering shaft 22 and the rack-and-pinion mechanism 25,there can be formed a free space. Such a space can be used for otherpurposes.

Because the two electric motors 41, 51 are connected via the single ballscrew 360′ to the rack shaft 26 c, no rack-and-pinion mechanism forconnecting the electric motors 41, 51 to the rack shaft 26 c isrequired. Consequently, there is no need for any rack guide other thanthe rack guide 70A. The thus arranged electric power steering apparatus400 is simple in construction.

The electric motors 41, 51 are connected through the belt transmissionmechanisms 341, 351 and the ball screw 360′ to the rack shaft 26 c. Thismakes it possible to provide a reduced distance between the rack shaft26 c and each of the motor shafts 41 a, 51 a. The electric motors 41, 51can also be positioned more freely.

The electric motors 41, 51 are well-known, less expensive ones ratherthan the aforementioned coaxial motors.

The electric motors 41, 51 extend in opposite directions and areparallel to the rack shaft 26 c. More specifically, the motor shaft 41 ahas the one end positioned in opposed relation to the one end of themotor shaft 51 a. The electric motors 41, 51 are disposed along theouter peripheral surface of the housing 61 c′. As can be seen from FIG.20, the first and second electric motors 41, 51 are positioned above therack shaft 26 c to thereby provide increased height above the ground.The thus arranged electric motors 41, 51 are protected from foreignobjects on the ground.

With the first and second electric motors 41, 51 positioned not in frontof but above the housing 61 c′, a vehicle body frame of the vehicle bodycan undergo plastic deformation in a stroke suitable for reducingcollision energy when the vehicle body collides with some objectpositioned forwardly thereof.

Referring to FIG. 21 through FIG. 23, there is shown an electric powersteering apparatus 500 according to a sixth embodiment of the presentinvention. The electric power steering apparatus 500 is the same as theelectric power steering apparatus 10 except that the assist torquemechanism 30 includes a fifth assist torque producing device 550 insteadof the assist torque producing device 50 as will be discussed below. InFIG. 21 to FIG. 23, the same components of the electric power steeringapparatus 500 as those of the electric power steering apparatus 10 areidentically numbered and their detail descriptions will be omitted.

As shown in FIG. 21, the assist torque mechanism 30 includes thesteering torque sensor 31, the control section 32, the first assisttorque producing device 40 and the fifth assist torque producing device550. The fifth assist torque producing device 550 includes the secondelectric motor 51 and the second worm gear mechanism 52.

Like the first assist torque producing device 40, the fifth assisttorque producing device 550 includes the second worm wheel 55 mounted onthe pinion shaft 24. This arrangement eliminates the need to provide theassist torque producing device 550 with the assist rack-and-pinionmechanism and assist pinion shaft as found in the second assist torqueproducing device 50 of the apparatus 10 in the first embodiment of thepresent invention.

The electric motors 41, 51 of the assist torque mechanism 30 of theelectric power steering apparatus 500 produce the assist torques to beapplied to the pinion shaft 24. Accordingly, the rack-and-pinionmechanism 25 transmits to a rack shaft 26 d the assist torques appliedto the pinion shaft 24 and the steering torque applied by the driver tothe steering wheel 21. No rack other than the rack 25 b is formed on therack shaft 26 d.

That is, the steering torque produced by the driver and the assisttorques produced by the first and second electric motors 41, 51 arecombined together to provide a composite torque. The composite torque istransmitted via the rack-and-pinion mechanism 25 to the rack shaft 26 dto thereby steer or turn the wheels 29, 29.

As is apparent from FIG. 21, the two electric motors 41, 51 are providedseparately from each other and connected to the pinion shaft 24. Thepinion 25 a is positioned between the electric motors 41, 51. The twoelectric motors 41, 51 produce the motive powers to be applied to thepinion shaft 24.

As shown in FIG. 22, the first electric motor 41 is positioned above therack shaft 26 d while the second electric motor 51 is positioned belowthe rack shaft 26 d.

As shown in FIG. 23, the electric power steering apparatus 500 includesthe pinion shaft 24, the rack-and-pinion mechanism 25, the steeringtorque sensor 31, the first and second worm gear mechanisms 42, 52 and ahousing 61 d. The housing 61 d has the first worm gear mechanism 42 andthe rack-and-pinion mechanism 25 housed therein. The housing 61 d hasupper and lower ends opened upwardly and downwardly, respectively. Theupper and lower ends of the housing 61 d are closed by the lids 65, 565,respectively. Reference numeral 568 denotes a bearing.

FIG. 23 provides a cross-sectional view of FIG. 4 taken along line 4B-4Bthereof to show in cross-section the first worm gear mechanism 42.

Also, FIG. 23 provides a cross-sectional view of FIG. 6 taken along line6A-6A thereof to shown in cross-section the second worm gear mechanism52 with the assist pinion shaft 57 replaced with the pinion shaft 24.

Adjacent the pinion shaft 24 for transmitting to the rack shaft 26 d thesteering torque applied to the steering wheel 21, the two electricmotors 41, 51 are provided separately from each other in such a manneras to provide the assist torques to the pinion shaft 24. The pinion 25 ais provided between the electric motors 41, 51. The electric motors 41,51 produce the motive powers to be applied to the pinion shaft 24.Because the electric motors 41, 51 are thus disposed compactly orclosely to the pinion shaft 24, the electric power steering apparatus500 are small in size.

With this arrangement, there is no need to connect any electric motor tothe rack shaft 26 d. There is no need to provide additional members forconnecting the electric motor to the rack shaft 26 d, either. Therefore,there can be formed a free space around the rack shaft 26 d. Such aspace may be used for other purposes.

As shown in FIG. 23, all the rack-and-pinion mechanism 25 and the twotransmitting members (i.e., the first and second worm gear mechanisms42, 52) for transmitting the motive powers produced by the first andsecond electric motors 41, 51, respectively, are mounted to the pinionshaft 24. This means that at the time the rack-and-pinion mechanism 25is mounted to the pinion shaft 24 with the electric motors 41, 51disposed adjacent the pinion shaft 24, inspection can be carried out onthe pinion shaft 24 to see if the pinion shaft 24 provides appropriateperformance. Thus, all the pinion shafts having undergone the inspectioncan provide substantially the same performance.

If the electric motor 51 were provided between the first electric motor41 and the pinion 25 a with the second worm gear mechanism 52 positionedbetween the first worm gear mechanism 42 and the pinion 25 a, therewould be provided an increased distance between the pinion 25 a and thefirst worm wheel 45 of the first worm gear mechanism 42 positioned at anupper level than the second worm gear mechanism 52. This undesirablyresults in an increased distance between the pinion 25 a and a distalend of the pinion shaft 24 connected to the universal joint 23.

For the electric power steering apparatus 500, however, the first wormwheel 45 can be positioned a reduced distance away from the pinion shaft25 a. More specifically, the distance between the pinion 25 a and thefirst worm wheel 45 with the second worm gear mechanism 52 positionedbelow the pinion 25 a is smaller than with the second worm gearmechanism 52 positioned above the pinion 25 a. This results in a reduceddistance between the pinion 25 a and the distal end of the pinion shaft24.

Providing such a reduced distance helps keep the pinion shaft 24 rigid.The electric power steering apparatus 500 has the advantage thatvibration occurred due to meshing engagement between the rack 25 b andthe pinion 25 a is prevented from being transmitted via the pinion shaft24 to the steering wheel 21.

Referring to FIG. 24 and FIG. 25, there is shown an electric powersteering apparatus 600 according to a seventh embodiment of the presentinvention. As will be described later, the electric power steeringapparatus 600 is the same as the electric power steering apparatus 100except that the assist torque mechanism 30 includes a sixth assisttorque producing device 640 instead of the first assist torque producingdevice 40. In FIG. 24 and FIG. 25, the same components of the electricpower steering apparatus 600 as those of the electric power steeringapparatus 100 are identically numbered and their detail descriptionswill be omitted.

As shown in FIG. 24, the electric power steering apparatus 600 includesthe third assist torque producing device 150 and the sixth assist torqueproducing device 640 serving the same function as the second assisttorque producing device 50 of the electric power steering apparatus 10.

The sixth assist torque producing device 640 includes the secondelectric motor 51, the second worm gear mechanism 52, the assist pinionshaft 57 and the assist rack-and-pinion mechanism 58.

The steering torque sensor 31 detects the steering torque applied to thesteering wheel 21 to produce the detection signal indicative of thedetection of the steering torque. The control section 32 produces themotor controlling signal on the basis of the detection signal. Thesecond and third electric motors 51, 151 produce the assist torquescorresponding to the steering torque on the basis of the motorcontrolling signal.

The assist torque produced by the second electric motor 51 can betransmitted via the coupling 56, the second worm gear mechanism 52, theassist pinion shaft 57 and the assist rack-and-pinion mechanism 58 to arack shaft 26 e. The assist torque produced by the third electric motor151 can be transmitted via the ball screw 160 to the rack shaft 26 e.The rack shaft 26 e has he racks 25 b, 58 b and the threaded portion 161formed thereon.

The steering torque the driver applies to the steering wheel 21, theassist torque produced by the electric motors 51 and the assist torqueproduced by the electric motor 151 can be combined together to provide acomposite torque. The composite torque is transmitted to the rack shaft26 e to thereby turn the wheels 29, 29.

The assist pinion shaft 57 is mounted or connected to the rack shaft 26e through the assist rack-and-pinion mechanism 58. The second electricmotor 51 produces the motive power to be applied to the assist pinionshaft 57 while the third electric motor 151 produces the motive power tobe applied to the rack shaft 26 e by means of the ball screw 160.

As shown in FIG. 25, the pinion shaft 24, the sixth assist torqueproducing device 640 and the third torque device 150 are mounted to ahousing 61 e.

The sixth assist torque producing device 640 is identical inconstruction to the second assist torque producing device 50 as shown inFIG. 5 and FIG. 6. FIG. 25 provides a cross-sectional view of FIG. 5taken along line 5A-5A thereof to show in cross-section the secondassist torque producing device 50, i.e., the sixth assist torqueproducing device 640.

FIG. 25 provides a cross-sectional view of FIG. 16 taken along line16A-16A thereof to show the pinion shaft 24, the rack-and-pinionmechanism 25 and the steering torque sensor 31.

As can be seen from FIG. 24 and FIG. 25, the assist pinion shaft 57 isconnected via the assist rack-and-pinion mechanism 58 to the rack shaft26 e connected through the steering shaft 22 to the steering wheel 21.The second electric motor 51 produces the motive power to be applied tothe assist pinion shaft 57 whilst the third electric motor 151 producesthe motive power to be applied to the rack shaft 26 e by means of theball screw 160 of small size. These electric motors 51, 151 arepositioned compactly or adjacent the rack shaft 26 e. The electric powersteering apparatus 600 having the thus compactly arranged electricmotors 51, 151 can be made small in size.

No electric motor is connected between the steering shaft 22 and therack shaft 26 e. Thus, there can be formed a free space around thesteering shaft 22 and the pinion shaft 24. Such a space may be used forother purposes.

The transmission member (i.e., the rack-and-pinion mechanism 25) fortransmitting to the rack shaft 26 e the steering torque applied to thesteering wheel 21 undergoes no motive power produced by the electricmotors. This arrangement helps the transmission member maintainsufficient strength. The steering torque, the motive power produced bythe electric motor 51 and the motive power produced by the electricmotor 151 are transmitted to different three portions of the rack shaft26 e. This arrangement helps the rack shaft 26 e maintain sufficientstrength.

In all the embodiments of the present invention as discussed withreference to FIG. 1 through FIG. 25, the pinion shaft 24 may be providedseparately from or independently of the steering shaft 22 instead ofbeing connected directly to the steering shaft 22. In such a case, thesteering torque sensor 31 detects a steering torque applied to thesteering wheel 22 to produce a detection signal indicative of thedetection of the steering torque. On the basis of the detection signal,any motive power source, for example, an electric motor other than theafore-mentioned electric motors produces a torque equal to the steeringtorque and then transmits the torque to the pinion shaft 24.

INDUSTRIAL APPLICABILITY

With the arrangement as explained above, the inventive electric powersteering apparatus includes two electric motors. These electric motorsare provided separately from each other. The two electric motors can befreely positioned within a space formed in a vehicle. The apparatus isuseful particularly in a large-sized vehicle.

1. An electric power steering apparatus comprising: a steering wheel; arack shaft to be connected to vehicle wheels, said rack shaft beingconnected to said steering wheel through a steering shaft and arack-and-pinion mechanism; a first electric motor for producing a motivepower to be applied to said steering shaft or between said steeringshaft and said rack-and-pinion mechanism; and a second electric motorfor producing a motive power to be applied to said rack shaft.
 2. Anelectric power steering apparatus comprising: a steering wheel; a rackshaft to be connected to vehicle wheels, said rack shaft being connectedto said steering wheel through a steering shaft and a rack-and-pinionmechanism; and two electric motors positioned adjacent and connected tosaid rack shaft through transmission mechanisms and a ball screw.
 3. Anelectric power steering apparatus comprising: a steering wheel; a rackshaft to be connected to vehicle wheels, said rack shaft being connectedto said steering wheel through a steering shaft, a pinion shaft and arack-and-pinion mechanism having a pinion formed on said pinion shaft;and two electric motors for producing motive powers to be applied tosaid pinion shaft, said two electric motors being connected to saidpinion shaft with said pinion positioned therebetween.
 4. An electricpower steering apparatus comprising: a steering wheel; a rack shaft tobe connected to vehicle wheels, said rack shaft being connected to saidsteering wheel through a steering shaft; a pinion shaft connected tosaid rack shaft through a rack-and-pinion mechanism; a first electricmotor for producing a motive power to be applied to said pinion shaft;and a second electric motor for producing a motive power to be appliedto said rack shaft through a ball screw.